Post-translational protein modification plays a pivotal role in selective protein functionalization for therapeutics, protein engineering, affinity design, and enzyme immobilization, among other applications.1 Within these, acyl carrier protein (ACP) labeling by 4′-phosphopantetheinyltransferase (PPTase) offers a highly versatile tool for site-selective covalent protein modification. Labeling of ACP fusion proteins represents one of the most flexible covalent protein labeling methods, as illustrated by orthogonal tagging with ACP peptides, bio-gel formation, and protein immobilization.2 This technique has also been successfully leveraged for visualization, isolation, functional, and structural studies of carrier protein-dependent biosynthetic enzymes.3 However, further advancement of these tools has been hampered by an inability to reverse this post-translational modification. Indeed, naturally occurring ACPs, often isolated in holo-form, may not be further modified selectively. Thus, there is a need in the art for reversibly labeling an ACP with a phosphopantetheine and with functionalized phosphopantetheine analogs. Provided herein are solutions to these and other problems in the art.